The present invention relates to semiconductor chips used in telecommunications, and in particular to mechanisms for switching between different frequency rates for different transmission protocols.
A variety of different semiconductor chips are used for specialized telecommunications applications. Examples of such chips include line transmitters, line receivers, framer ICs, etc. Such devices often are designed to support more than one communications protocol. For example, they may support both E1 and T1, or may support E3, STS-1, DS3, etc. When a customer buys such a chip and places it in a particular application, the protocol for which it is being used must be selected. That protocol typically corresponds to a different frequency rate.
Depending on which protocol is selected, a number of internal functions are performed differently. The most basic is that the frequency of transmission, or bit rate, will vary from one protocol to another. Another difference is that the protocol will specify parameters for the pulse shape of the bits, which will require different approaches on shaping the pulse for transmission or detecting it upon reception. In addition, the framing of the bits can vary as well.
A couple of different methods are used for indicating which protocol, or frequency rate, is to be used. One is simply to have a toggle switch or a separate control input pin which indicates which protocol. In an alternate approach, the chip can figure out the protocol from the frequency of the clock applied to the chip in connection with the data. This is accomplished by including a second clock input which can be used to compare against the protocol clock to determine its frequency and thus identify its protocol.
The method of comparing two frequency sources using a digital frequency discriminator is well know. However, this method requires not only an additional accurate clock source, the second clock source adds clock noise to the system. It is impossible to utilize this approach if the application cannot afford or does not have the additional independent clock source.
The present invention provides a method and apparatus for automatically determining the protocol being used from the frequency of an applied clock without the need for a separate pin or switch or a second external clock. The clock""s frequency is identified when its frequency falls into the set range for which the apparatus is targeted. Based on the detected frequency in the set range, a mode select signal is generated. The mode select signal causes the chip to configure to the appropriate frequency for that mode, as well as any other unique configuration parameters.
In one embodiment, the invention generates a ramp signal triggered by the external clock (which is the clock frequency for the desired protocol). The clock is simultaneously applied to a counter. When the ramp signal reaches a reference voltage, the count of the counter is compared to at least one threshold to determine to which frequency it corresponds. In response to this determination, the chip is configured according to the communication mode or protocol indicated. This may be simply the appropriate frequency, but, depending upon the chip, may include pulse shaping, framing and other configuration selections.
In one embodiment, the ramp signal is generated using a fixed current source which charges up a capacitor. Two switches are connected in parallel with the capacitor for discharging the capacitor, one switch being a smaller current device than the other switch. The purpose of the two switches is to allow staggered turn on and turn off of the ramp to avoid voltage spikes which could cause an inaccurate start of the ramp signal.
In one embodiment, the fixed current source is connected to an off-chip resistor. This resistor, in combination with the internal trimmable bandgap reference generator, ensures stable and accurate ramp timing over variations in temperature and power supply operation. During manufacture of the chip, a known clock frequency can be applied and the time for the ramp to reach the reference voltage can be determined. If the time is off, the bandgap reference voltage is trimmed accordingly to adjust the current source and thus the targeted count in order to compensate for process variations.
For a further understanding of the nature and advantages of the invention, reference should be made to the following description taken in conjunction with the accompanying drawings.